1. Field of the Invention
The present invention relates to an infrared absorbent for use in a thermal type infrared sensor, wherein the temperature of the temperature sensing part having a thermally insulated structure rises by the incidence of an infrared ray, and relates to a method for forming the absorbent.
2. Description of the Related Art
Among the infrared sensors, those classified as thermal type have a mechanism that the temperature of the temperature sensing part comprising a thermally insulated structure rises by the incidence of an infrared ray, and the temperature rise is converted into an electric signal by utilizing the pyroelectric effect, the thermoelectric effect, or the temperature coefficient of a resistor. With such a thermal type infrared sensor, it is necessary for realizing an increased sensor sensitivity to increase the temperature rise of the temperature sensing part by effectively absorbing the incident infrared ray, converting it into heat, and transferring it to the temperature sensing part. For this purpose, an infrared absorbent is placed over the temperature sensing part, where the incident infrared ray is converted into heat. Generally, a vapor deposition film of a metal such as Au black or NiCr, or a printed film of a material such as a polymer with a high infrared absorption coefficient is used as this absorbent. Regarding the patterning of this infrared absorbent, the metal mask method, the lift-off method or the like is used. For example, infrared absorbents (blackbodies) described in Japanese Unexamined Utility Model Application Publication No. 3-117739 and Japanese Unexamined Utility Model Application Publication No. 4-59427 were obtained by patterning according to such a method.
However, when deposition/patterning of an infrared absorbent is performed according to the above-described method, an absorbent pattern with a uniform film thickness results, and the following undesirable problem occurs: when an infrared absorbent formed with a metal having a good thermal conduction such as Au has a uniform film thickness, the heat transfer route through which heat flows into an external part (heat sink) from the central region and its neighborhood of the absorbent pattern via the edge region of the absorbent pattern has a large heat conductance, resulting in a large heat loss in this portion so that the temperature rise of the temperature sensing part becomes small. FIG. 3 shows the heat transfer route in this case. In this figure, the numerals 1, 2, and 3 represent a temperature sensing part, an infrared absorbent (blackbody), and a heat sink, respectively. Heat diffuses along route A from the central region and its neighborhood of the infrared absorbent 2. However, regarding the route A, the route passing through the infrared absorbent 2 has a good thermal conduction, with a result that the heat conductance of this portion becomes large, restricting temperature rise of the temperature sensing part 1.
Accordingly, it is an object of the present invention to reduce the heat loss in order to realize an efficient temperature rise of the temperature sensing part, by devising the form of a deposited infrared absorbent film.
According to an infrared absorbent of the present invention, the film thickness of the edge region of the infrared absorbent pattern formed on the surface of a temperature sensing part is smaller than that of the central region of the pattern. With this form of the infrared absorbent, the heat conductance of the heat transfer path through which heat flows into a heat sink from the central region and its neighborhood of the absorbent via the edge region of the absorbent becomes smaller, thus making it possible to have a temperature sensing part with a larger temperature rise by virtue of a smaller heat loss from the temperature sensing part. The film thickness in this case is preferably varied by forming an inclination from the central region and its neighborhood of the pattern toward the edge region of the pattern. However, the film may decrease its level step by step, or it may have some steps. It is noted that the central region of the pattern refers to a region including at least the center of the pattern, and may be limited to the center, or may include its peripheral portion.
With this configuration of the infrared absorbent, a heat loss from the edge region of the infrared absorbent is reduced, and it follows that it is possible to increase the area of the infrared absorbent. Subsequently, a further temperature rise of the temperature sensing part can be achieved as a result of the increased amount of the incident infrared ray in proportion to the increased area.
Mask vapor deposition method can be applied for the deposition of the above-described infrared absorbent. A metal mask is usually used for the mask. The mask is formed so that the part of the mask facing the central region and its peripheral region of an infrared absorbent pattern to be formed on the surface of a temperature sensing part has an opening, and the mask has a form having a slope inclined from the edge of the opening toward the edge region of the pattern. This mask is placed over the temperature sensing part for depositing an infrared absorbent on the temperature sensing part by vapor deposition or sputtering.
Another type of mask is formed so that the part of the mask facing the central region and its peripheral region of an infrared absorbent pattern to be formed on the surface of a temperature sensing part has an opening, the mask has a substantially plate shape extending from the edge of the opening toward the edge region of the pattern, and the mask is arranged over and spaced from the temperature sensing part. Thus, an infrared absorbent is deposited on the temperature sensing part by vapor deposition or sputtering. According to the deposition method described first, the absorbent is deposited along the slope of the metal mask, with a result that the film thickness of the completed absorbent is inclined from the central region and its neighborhood of the pattern toward the edge region of the pattern. According to the latter method, deposition occurs also at the edge region of the pattern by the wraparound effect at the time of deposition. As a result, an absorbent is formed, the absorbent having a form wherein the film thickness of the edge region of the pattern is smaller than that of the central region of the pattern.
According to the present invention, by making the edge of an infrared absorbent pattern thinner, the heat conductance of the heat transfer route through which heat flows into the exterior part from the central region and its neighborhood of the absorbent via the edge region of the absorbent becomes smaller, thus making it possible to have a temperature sensing part with a larger temperature rise by virtue of a smaller heat loss from the temperature sensing part. Also since the heat loss from the edge region of the above-described absorbent becomes smaller, it is possible to increase the area of the absorbent. Therefore, further temperature rise of the temperature sensing part can be achieved by the increased amount of the incident infrared ray in proportion to the increased area.